| Methyl lactate is an important chemical product, which is widely used in medicine, resincoatings, adhesives, cleaning solvents, dry-cleaning fluid, printing ink and other field.Traditionally, methyl lactate is synthesized by esterification of lactic acid and alcohols usingsulfuric acid as catalyst. In this process, there are raw materials (lactic acid) complex productionprocess, the reaction by product, sulfuric acid corrosion of the equipment and seriousenvironmental pollution problems. Hence, it is a very interesting research topic to develop newenvironmentally friend route to produce methyl lactate. Recently, it was reported thathetero-atom (Me) replaced the zeolite could catalyze dihydroxyacetone (or glyceraldehyde)isomerization-esterification of methyl lactate. Among them, Sn-doped Beta zeolite showedrelatively high catalytic activity and selectivity.In this thesis, the catalytic performance of Sn-doped Beta zeolite (Sn-Beta), Ti-dopped Betazeolite (Ti-Beta) and Sn-doped SBA-15zeolite (Sn-SBA-15) catalysts, synthesized byhydrothermal method, were investigated in the reaction of dihydroxyacetoneisomerization-esterification of methyl lactate. The influence of various factors, such as the typesand the doping concentration of heteroatoms, and the reaction conditions (reaction temperature,catalyst amount, reaction time) on the catalytic performance was studied; Combined with avariety of characterization methods, the composition, structure and surface properties of thezeolite catalysts have been studied and associated with the performance of the catalyst. The mainresearch contents and the conclusions are summarized as follows:1. All the Sn-Beta zeolites with different Si/Sn ratio (Si/Sn=90、100、130、200) could act asactive catalysts for the isomerization-esterification reaction of dihydroxyacetone to methyllactate. With the increase of Si/Sn ratio, the catalytic activity of the zeolite decreased. The zeolitewith Si/Sn=90showed the highest activity and yield of methyl lactate, a98.0%conversion ofdihydroxyacetone with a93.3%yield of methyl lactate is obtained after20h reaction at areaction temperature of160℃. In addition, the catalytic activity of Sn-Beta zeolite remainedunchanged after five recycling tests, showing a good recyclability and stability. Thecharacterization results of XRD and UV spectroscopy confirmed that part of the Sn atomsexisted in the zeolite framework in the form of tetrahedral coordination. Compared with otherSn-Beta zeolites, the sample with Si/Sn=90possesses more Lewis acid center, which might berelated to the excellent catalytic performance of the catalyst.2. The catalytic properties of Ti-Beta zeolite with different Si/Ti ratio (Si/Ti=62.5,90,125)was studied. The results showed that the zeolite activity decreases gradually with the increase of Si/Ti ratios. The zeolite with Si/Ti ratio of62.5exhibited relatively high catalytic activity. Underthe optimized conditions, the conversion of dihydroxyacetone was60.2%, while the yield ofmethyl lactate was55.8%. In addition, Ti-Beta zeolite catalysts showed good recyclability andstability. The characterization results of XRD and UV spectroscopy confirmed that part of thetitanium atoms is present in the zeolite framework in the form of tetrahedral coordination.NH3-TPD results showed that the zeolite with Si/Ti ratio of62.5has a relatively strong Lewisacidity and more acidic sites, thus showing relatively high catalytic activity.3. Sn-SBA-15materials with different Si/Sn (Si/Sn=10,40,60,80) were synthesized byhydrothermal method, and their catalytic performance for the dihydroxyacetone isomerization-esterification reaction to produce methyl lactate was investigated. The results showed that thepure SBA-15(without introducing Sn atoms) is active catalyst for the title reaction withrelatively low catalytic activity; while the catalytic performance of Sn-SBA-15is much betterthan that of pure SBA-15. With the increase of Si/Sn ratio, the catalytic activity of Sn-SBA-15materials decreases gradually. The Si/Sn=10sample shows excellent catalytic performance underthe optimal reaction conditions (reaction temperature was90℃, reaction time was6h), theconversion of dihydroxyacetone is close to100%, and the yield of methyl lactate reaches95.9%.A variety of characterization results show that a small part of the Sn atoms can enter into theframework of SBA-15, while most of Sn atoms should be existed in the form of highly dispersedtin oxide species located on the surface and the pores of SBA-15. The existence of relativelystrong Lewis acidity and more acid sites, should be mainly responsible for the high catalyticactivity of Sn-SBA-15with Si/Sn ratio of10. |
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